1. Field of the Invention
The present invention relates generally to a device and method for accurately determining volumetric blood flow and, more particularly, to devices and methods utilizing ultrasonic transducer equipped arterial catheters configured to measure absolute or total vascular flow. The devices of the invention are particularly useful in measuring total pulmonary artery flow in segmental arteries, total main or segmental arterial and venous flow in other visceral organs, such as left or right main coronary, carotid, subclavian arteries or veins or renal blood flow in main or segmental arteries or veins.
2. Discussion of the Related Art
For many years diagnostic catheters have been available which measure cardiac output and pressures utilizing dye (contrast agent) dilution, thermodilution or oxygen consumption as the basis for measurement. More recently, catheters with more benign approaches have been developed which utilize Doppler ultrasonic transducers to measure instantaneous flow velocity in coronary and other arteries. These devices measure the Doppler shift or frequency modulation caused by the movement of blood cells within the blood vessel in known directional relation through an ultrasonic beam. In order for volumetric flow within the vessel of interest to be accurately determined utilizing such Doppler shift systems, however, such parameters or independent variables as the area of the vessel, velocity profile and the angle between the ultrasonic beam and the direction of flow must be determined and controlled with the required degree of accuracy.
One approach to the solution of this problem involves the use of a catheter device in which a relatively large, uniform beam of ultrasonic energy using a toroidal ultrasonic transducer carried by the catheter which emits a substantially uniform beam to illuminate the entire vessel of interest. This device attempts to make volumetric flow measurements independent of the size of the vessel, angle of incidence between flow and the ultrasonic beam, the velocity profile within the vessel, or the exact location of the transducer within the vessel. While such measurements are possible, the compromises involved reduce the precision and accuracy of the measurements below desired levels.
In addition, blood flow measuring catheter systems utilizing smaller single Doppler shift ultrasonic transducers have also been proposed. Examples of such devices which utilize the same ultrasonic transducer to measure both the diameter of the vessel of interest and the velocity of the blood cells flowing therethrough have also been proposed by Segal in U.S. Pat. Nos. 4,733,669 and 4,856,529. See also Segal et al "Instantaneous and Continuous Cardiac Output Obtained with a Doppler Pulmonary Artery Catheter", Journal of the American College of Cardiology, Vol. 13, No. 6, May 1989: 1382-92.
The accuracy of these devices depends on the precision of placement of the crystal in the lumen of the vessel of interest to achieve the proper known (assumed) angle between the direction of flow and the direction of the ultrasonic beam, and on the location of the ultrasonic transducer with respect to the wall of the vessel for accurate diameter determination. In addition, the crossection of the vessel must be assumed to be a constant function of the diameter.
Systems using multiple ultrasonic transducers in which one or more of such transducers are used to measure the flow velocity and one or more additional transducers are used to determine the diameter of the vessel of interest are illustrated and described by Nassi et al (U.S. Pat. Nos. 4,947,852 and 5,078,148). A Doppler pulmonary artery catheter using multiple transducers is illustrated and described by Segal et al in "Instantaneous and Continuous Cardiac Output in Humans Obtained with a Doppler Pulmonary Artery Catheter", Journal of the American College of Cardiology, Vol. 16, No. 6, Nov. 15, 1990:1398-407.
Other methods and ultrasonic devices based on the Doppler principle have been extensively applied in the measurement of coronary blood flow. For early use see, for example, Bing et al, "Techniques to Measure Coronary Blood Flow in Man", American Journal of Cardiology, Vol. 29, June, 1972:75-80 and Benchimol et al, "New Method to Measure Phasic Coronary Blood Velocity in Man", American Heart Journal, Vol. 81, No. 1, January, 1971:93-101. Devices of this type are described in Kohl et al, "The Pulsed Doppler Coronary Artery Catheter",Circulation, Vol. 56, No. 1, July 1977. A device for measuring blood flow and relatively small coronary artery branches is described by Wilson et al, "Transluminal Subselective Measurement of Coronary Artery Blood Flow Velocity and Vasodilator Reserve in Man", Circulation, Vol. 72, No. 1, July 1985:82-92. A steerable Doppler catheter having a central distal guidewire is described by Sibley et al in "Subselective Measurement of Coronary Blood Flow Velocity Using a Steerable Doppler Catheter", Journal of the American College of Cardiology, Vol. 8, No. 6, December 1986:1332-40.
While all of these prior devices and techniques have achieved a certain degree of success, each in its individual application, it is axiomatic that each also introduces certain measurement inaccuracies owing to the fact that certain assumptions must be made with respect to one or more independent variables involved in the volumetric flow calculations which are subject to time-variable changes. Thus, for example, one must assume that a vessel is always round or of constant crossection or that the ultrasonic transducer remains at a substantially constant angle with respect to the flow through the vessel of interest, etc. If dependence on a need to make any or all such assumptions in the determination can be eliminated, the accuracy and usefulness of such devices would be greatly enhanced.
There are many reasons why such measurements are important, and increased accuracies in them could provide valuable diagnostic tools in many areas. One important area in which the accurate measurement of absolute volumetric or total blood flow is important is in the evaluation of pulmonary hypertension. It is clinically important to know whether pulmonary hypertension is fixed or variable in those patients with primary or secondary pulmonary hypertension as well as pre- and post-cardiac and lung transplant patients and in those with adult respiratory distress syndrome.
In this regard, primary pulmonary hypertension is a serious and widespread condition which has been difficult to conclusively diagnose. The disease primarily affects small pulmonary arteries (40-300 .mu.) resulting in intimal/medial hypertrophy fibrosis or subsequent loss of pulmonary capillaries. These histologic changes result in elevated pulmonary resistance leading to pulmonary hypertension. Pulmonary hypertension is a significant cause of morbidity and mortality in females between the ages of 20 and 40.
Prior to the present invention, it has been difficult to separate those patients with fixed and variable pulmonary hypertension. To this end, diagnostic trials have been instituted with various vasodilator drugs that include PGI.sub.2, PGE.sub.2, nifedipine, verapamil, hydralazine, nitroprusside, phentolamine, diazoxide, nitroglycerin, etc. Pulmonary resistance is calculated before the after administration of a vasodilating drug according to variations in electrical resistance measurements using thermodilution cardiac output and mean pulmonary artery (or pulmonary artery-wedge gradient) pressure. Even slight mistakes in measuring either of these two independent variables, however, results in significant error in calculation and inconclusive results. The primary reason for this effect apparently is that when a vasodilating drug is given systemically, it does not alter just the pulmonary vasculature, but it also affects systemic pre-load and after-load. Consequently, one may never be sure whether the measured drop in resistance is a consequence of reduced pulmonic pressure (secondary to systemic vasodilation), reduced cardiac contractility or increased cardiac output.
Furthermore, large amounts of the drug are required in these provocative studies, and this frequently makes the patient ill, at times resulting in significant morbidity and even death. This occurs as a consequence of dropping the systemic pressure in venous return to the right heart that is pumping against a fixed resistance in the pulmonary system.
There are many important advantages in being able to measure absolute segmental pulmonary artery flow as compared to measuring whole lung flow with thermodilution cardiac output. The ability to measure segmental flow with high accuracy would permit evaluation of regional changes in resistance while systemic hemodynamics remain unchanged. This isolation and localization means that only a fraction of the systemic vasodilating dose need be given into the distal vascular bed to measure reactivity. Thus, in this manner, the procedure would become a great deal safer inasmuch as it does not involve perturbing systemic hemodynamics with the vasodilator challenge.
It is further contemplated that catheters of the class would also be applicable to measuring absolute segmental visceral organ flow such as that pertaining to the renal, splenic and other arteries. In addition to local or segmental flow, absolute large vessel flow (i.e., subclavian artery, main renal artery, right coronary artery, left main and carotid artery flow, for example) would be readily measurable.
Accordingly, it is an object of the present invention to provide a self-aligning ultrasonic Doppler shift flow measuring catheter which measures absolute volumetric flow through a vascular lumen of interest.
It is a further object of the invention to provide a flexible ultrasonic Doppler flow catheter which eliminates the need for precisely posturing the catheter in the vessel of interest.
A still further object of the present invention is to obviate the need for measuring the diameter of the vessel of interest and/or estimating the crossectional area thereof.
Another object of the invention is the provision of a method and apparatus for measuring absolute large vessel flow.
Another object of the invention is the provisions of a method and apparatus to measure absolute segmental visceral organ blood flow.
These and other objects will become apparent based on the description and accounts of the invention including the detailed description and drawings below.